1. Field of the Invention
The present invention relates to a multichannel SQUID flux measuring apparatus for detecting a magnetic field of an object of flux measurement by measuring flux emitted from the object of flux measurement, such as brain or heart, by use of a plurality of SQUIDs (superconducting quantum interference devices), and more specifically, to a multichannel flux measuring apparatus having a function for compensating an interference due to a crosstalk between a plurality of channels.
2. Description of the Related Art
A method of detecting a magnetic field of an object to be measured, that is, so-called biomagnetic measurement, is known as a method of detecting an internal state of a human living body from outside. Regarding the biomagnetic measurement, since the intensity of the flux radiated from the living body is one ten thousandth to one hundred millionth of that of terrestrial magnetism (0.5.times.10.sup.-4 T (Tesla)), there is a demand of a technique for detecting a very weak flux. Recently, as a technique for detecting such a very weak flux, a flux measuring apparatus using a SQUID (superconducting quantum interference device), which is an application of the superconducting electronics, is brought into focus.
FIG. 1 is a diagram showing the main structure of a conventional multichannel SQUID flux measuring apparatus using a plurality of the above-described SQUIDs.
As shown in FIG. 1, the conventional SQUID flux measuring apparatus includes a plurality of fluxmeters 1 (the n number in FIG. 1) (1-1 to 1-n), a signal processing circuit 2, a data collecting portion 3, a display 4 and a data analyzing portion 5. Each of the magnetic fluxmeters 1 (1-1 to 1-n) has a corresponding one of pick-up coils 11 (11-1 to 11-n), a corresponding one of SQUID chips 12 (12-1 to 12-n) and a corresponding one of FLL (flux locked loop) circuits 13 (13-1 to 13-n). For example, the fluxmeter 1-1 includes the pick-up coil 11-1, the SQUID chip 12-1, and the FLL circuit 13-1. That is, the magnetic measurement apparatus shown in FIG. 1 is an n-channel SQUID fluid measuring apparatus in which each of the fluxmeters 1-1 to 1-n serves as a unit channel.
The pick-up coils 11-1 to 11-n are, for example, as shown in FIG. 2A, two-dimensionally arranged at a predetermined interval so as to cover the surface of an object P of flux measurement (head portion of a patient). It should be noted that it is seen in FIG. 2A that pick-up coils 11-1 to 11-n are arranged in line since this figure is a side view; however, in reality, they are arranged in a curved-surface manner and in a two-dimensional array manner so as to cover the surface of the object P of flux measurement, as shown in FIG.
The SQUID chips 12-1 to 12-n of the fluxmeters 1-1 to 1-n each has an input coil, a modulation coil and the like. The power to these coils is controlled by the corresponding FLL circuit 13, and the corresponding SQUID chip 12 is driven.
In each of the fluxmeters 1-1 to 1-n, the flux (flux density) which are emitted from the object P of flux measurement and are linked within the pick-up coil 11, is converted into a corresponding electrical signal (analog signal) by the SQUID chip 12 driven by the FLL circuit 13. The converted electrical signal is output from the FLL circuit 13. Electrical signals output from the fluxmeters 1-1 to 1-n, that is, electrical signals output from the FLL circuits 13-1 to 13-n are all sent to the signal processing circuit 2.
The electrical signals output from the fluxmeters 1-1 to 1-n are treated in the signal processing circuit 2 so that they can be sent to the data collecting unit 3, and then the signals are sent to a data collecting unit 3. The data collecting unit 3 includes an A/D (analog/digital) converter 31, a data recording section 32 and the like, and the input electrical signals (analog signal) are converted into digital data by the A/D converter 31. The converted digital data are recorded/saved in a non-volatile recording medium such as a hard disk in the data recording section 32.
The digital data recorded on the non-volatile recording medium are displayed directly on a display 4 in the form of a magnetic field distribution or time waveform, and once transferred to the data analysis section 5. The data transferred to the data analysis unit 5 are subjected to an analyzing process, for example, the detection of the distribution of one or more current sources in a living body, and then the data are displayed on the display 4.
In such a conventional multichannel SQUID flux measuring apparatus, a great number of, for example, 100 or more, SQUID fluxmeters each consisting of a pick-up coil, a SQUID chip and an FLL circuit and serving as a unit channel, are provided. With this structure, magnetic fields of a great number of points to be measured are measured at the same time in measurement of biomagnetic field (for example, brain or heart).
However, the above-described multichannel SQUID flux measuring apparatus has a great number of pick-up coils and SQUID chips, and therefore inter-channel magnetic interactions occur in the pick-up coils, the wiring connecting pick-up coils and SQUID chips, and the SQUID chips. That is, a signal flowing in an arbitrary channel and a linking flux interfere a signal flowing in another channel, and in reverse, a signal flowing in another channel and linking flux interfere the arbitrary channel. Such a phenomenon is called crosstalk, and a crosstalk causes an undesirable error in measurement of magnetic field.
The main factor which causes such a crosstalk is a magnetic interaction between pick-up coils. In the conventional multichannel SQUID flux measuring apparatus, the distance between pick-up coils of channels is rendered long in order to lessen the magnetic interaction. However, in the case where the area to be measured (measurement range) is constant, it is required to reduce the number (channel number) of pick-up coils or reduce the coil diameter of each pick-up coil for increasing the distance between the pick-up coils. When the number of pick-up coils is decreased, the number of measurement points provided in the measurement area is accordingly reduced, thus lowering the accuracy of the magnetic field measurement. When the coil diameter of each pick-up coil is reduced, the coil area of each pick-up coil with which flux links, is reduced, thus lowering the accuracy of the flux measurement of each coil (the signal-to-noise ratio, that is, S/N ratio is deteriorated).
As described above, in the case of the conventional multichannel flux measuring apparatus, if some measures are taken to lessen the interference due to crosstalk, some other problems than the interference due to the crosstalk are created. As a result, the measures taken are always insufficient.